In order to show a color image, conventional displays have three primaries: red, green and blue. In Europe, the chromaticity coordinates of these three primaries are defined by the EBU standard for standard definition content, and by the Rec709 standard for high definition content. The chromaticity coordinates are also referred to as color points or colors. In other countries, different standards may define the colors of the three primaries. The chromaticity coordinates of the primaries in the EBU standard are in the CIE 1931 xy-color space for red: x=0.64 and y=0.33, for green: x=0.29 and y=0.60 and for blue: x=0.15 and y=0.06. The white point of the display is also defined in the EBU standard and is close to D65 (6500K) which has the chromaticity coordinates: x=0.3127 and y=0.3290. In practice, display manufactures may select another white point than D65. Many display apparatus have a somewhat more blue white point, for example, 8600K or even 10000K. This set of primary and white point colors determines the color behavior of the display. When using EBU primaries, white D65 is obtained by a ratio of the luminance's of the red, green and blue primaries of 22:71:7.
Such a conventional display with the above discussed three EBU primaries cannot reproduce all visible colors. Only the colors within the color triangle of which the primaries are the vertices can be displayed, or stated more accurately, the colors within the 3 dimensional display gamut can be displayed. Or said differently the color gamut of the EBU display is defined as the area in the chromaticity space within this triangle including its borders. Consequently, especially very saturated colors like the monochromatic colors cannot be reproduced. With monochromatic color is meant a color with a single peak at a particular wavelength in the visible light spectrum.
Recently, so-called wide gamut displays are getting more and more attention. These displays have primaries which are selected to obtain a larger color gamut than that of the EBU standard primaries. Hence, the wide gamut displays are able to produce colors which are more saturated than an EBU display. In a wide gamut LCD display the three primaries for the wide gamut may be obtained by adapting the spectrum of the backlight and/or changing the transmission band of the color filters. Alternatively, or additionally, a further primary may be added inside or outside the triangle but preferably within the visible range of colors. Wide gamut displays with 3 primaries are sometimes called enhanced gamut displays, and with more than 3 primaries, multiprimary displays.
Most content is coded within the gamut defined by the EBU primaries (i.e. the television camera records scenes according to this gamut, and most natural colors are not too saturated). For wide gamut displays, the display gamut may be very different from this input gamut. If the input image signal, which usually is an RGB signal, is used directly to drive the wide gamut display the color reproduction is distorted and for most images not preferred above the color reproduction of the usual EBU gamut. A better color reproduction can be achieved if a color mapping is applied to the input image signal extending the EBU gamut to the wide gamut before driving the wide gamut primaries. However, the known color mappings, which typically implement a saturation increase, still cause a non-optimal color reproduction on the wide gamut displays.
Similarly, if EBU content has to be displayed on a display with color primaries which provide a gamut smaller than the EBU gamut, or if a wide gamut content has to be displayed on a display with an EBU gamut, the color mapping has to shrink the input gamut to a smaller display gamut, and again the color reproduction will be impaired.